Two hypotheses which attempt to explain fetal escape from maternal allograft rejection are being tested. The first suggests that profound maternal immunosuppression allows allograft maintenance. The second proposes that fetal immunoregulatory functions suppress locally active maternal lymphocytes. We are testing the immunocompetence theory by conducting a detailed investigation of the in vivo immune responses elaborated by mice at specific times during pregnancy to a panel of selected thymus dependent and thymus independent antigens. The second hypothesis is being tested by observing several characteristics of the in vitro interactions between adult lymphocytes and fetal and/or neonatal lymphohematopoietic cells.